In the field of optical position detectors, two types of detectors are in common use and each has problems peculiar to its design. The first of these detectors is commonly known as an absolute encoder. "Absolute" referring to the ability of the encoder to recognize the position of a movable element without referring to a base, home, or starting position. These absolute encoders typically include a scale with alternating opaque and optically transmissive sections arranged in a particular pattern along a series of tracks. The scale is movable along a path which positions each of the tracks intermediate a light source and corresponding photodetectors. The opaque and transmissive sections control the state of the photodetectors by either blocking or passing light resulting in a corresponding off or on condition of the photodetectors. The on/off status of the photodetectors correspond to a 1/0 condition such that a combination of all the photodetectors forms a unique binary number indicative of the position of the scale. The pattern of the opaque and transmissive sections are typically arranged in a gray or straight binary code depending upon the complexity and desired accuracy of the encoder. However, while the choice of the coding scheme has some influence on the accuracy of the encoder, the maximum degree of accuracy is ultimately a function of the number of tracks and the magnitude of travel. Assuming a linear encoder is to be designed which requires a length of travel (L) and physical packaging restraints limit the number of tracks to (N). Then the equation: EQU degree of accuracy=L/2.sup.n
determines the maximum achievable accuracy. For example, an element movable through a stroke of 5 cm requiring a degree of accuracy of .+-.1 MM must include six tracks. ##EQU1## Eliminating tracks from the exemplified encoder reduces the accuracy of the system by a factor of two for each track removed. As one might guess, as the number of tracks increases, the accuracy increases, as does the cost, complexity, and physical package size of the encoder For systems requiring long travel or high accuracy, the absolute encoder would need a large number of tracks resulting in the encoder becoming unwieldy and difficult to physically locate.
Alternatively, a second type of encoder is commercially available and commonly referred to as an incremental encoder. These incremental encoders overcome many of the shortcomings of the absolute encoders, but in so doing introduce a number of problems unique to their design. Incremental encoders ordinarily include a single track of alternating opaque and optically transmissive sections arranged along a scale so as to be sequentially introduced between a light source and a photodetector, affecting the state of the photodetector in much the same way as in the absolute encoders. Because the incremental encoders include only a single track, a unique multiple digit binary number indicating absolute position cannot be formed, but position can be determined relative to some preselected starting point simply by counting the number of opaque and transmissive sections passing the photodetector. Obviously, some provision for detecting direction of travel must be included in the design to allow the count to be consistently incremented or decremented relative to the direction of travel. Accuracy of incremental encoders is controlled by the width of the opaque and transmissive sections which can be made extremely fine and limited only by the ability of the photodetector to recognize a change from an opaque to a transmissive section. Therefore, accuracy of an incremental encoder may be greatly enhanced without a corresponding increase in physical size or complexity.
However, the shortcoming of this type of encoder lies in the inherent inability to calculate absolute position without referring to a known starting position. This requirement forces the device to be returned to a starting position during an initial start up period. After a period of power loss, the encoder is hopelessly lost until returned to the starting position.
For example, position encoders are typically employed for detecting governor position of an engine. Using an incremental encoder can give the accuracy necessary to provide stable engine control, but each time the engine is started, the governor must be initially positioned at some preselected starting location. This is an inconvenience to the operator; however, if during actual operation, electrical power is momentarily lost, then the incremental encoder will not indicate movement of the rack which has occurred and will result in false position information being provided to the governor control. If the error is significant, engine operation outside of a preferred range could occur and cause damage or even failure of the engine or drive train.
Further, as the accuracy of the device is increased, the ability of the electronic circuitry to detect movement and direction becomes increasingly difficult and susceptible to error. For example, switching noise ambient light, or RFI interference could mask a change from opaque to transmissive and result in an occasional missed count. While the accuracy of the system is high, a single missed count will have only a small effect on accuracy; unfortunately, the errors are cumulative. This cumulative error becomes larger the longer the device is operated and can only be compensated for by periodic calibrations during operation. The operator must periodically return to the starting position to remove the cumulative error. This is an annoyance to the operator and in some applications an impossibility.
The present invention is directed to overcoming one or more of the problems as set forth above.